Tree box filters, which are also known as tree box planters, function as bioretention systems for managing and filtering stormwater runoff. They are a well-known Low Impact Design (“LID”) approach to mitigating the impacts of hydromodification, impervious surfaces, and the general alteration of flow, and to managing the flow of stormwater runoff on developed land. Bioretention systems utilize soils and both woody and herbaceous plants to remove pollutants, including ultra-fine and dissolved pollutants, from stormwater runoff close to their source. The systems mimic the natural (i.e., pre-development) stormwater flow from the land.
Tree box filters can be used to treat stormwater that has run over impervious surfaces at commercial, residential, and industrial areas, such as in parking lot islands, roads, and median strips. In a conventional tree box filter, stormwater runoff flows into an in-ground or above ground vault-shaped container filled with bioretention media, including mulch and engineered soil. The container also includes a native, non-invasive tree, shrub, or other vegetation. Runoff can be diverted to the bioretention area, either directly or by a curb or gutter system.
As water infiltrates through the tree box filter and the tree box filter temporarily stores runoff water, the bioretention media captures particulate matter, including ultra-fine and dissolved pollutants, and allows the treated stormwater to percolate through the system. Filtered runoff can sometimes be collected in an underdrain, which consists of a perforated pipe in a gravel layer installed along the bottom of the filter bed. The stormwater eventually exits through one or more outlet openings in the container into a drainage system or water retention/storage system.
One of the concerns that has emerged is the ability of tree box filters to accommodate infiltration. In areas suitable for infiltration, tree box filters can be designed to allow runoff to infiltrate into the surrounding soils from the ground surface. Infiltration can be desirable, for example, in areas with permeable surrounding soils. Stormwater exits the vault-shaped container through an open bottom structure that allows for infiltration to the subgrade. The tree box filter's open bottom structure, however, limits the ability of the vault-shaped container to hold the bioretention media, gravel, and other container materials within the vault, before it is delivered and installed at a given site or landscape area. Additional space is thus needed to store the materials before delivery and installation. Because separate site visits may be required to install the media, additional space, coordination, time, fuel, labor, and other resources may be required to transport the materials during delivery of the system. Additional time and labor may also be needed to separately load, unload, and carry the bioretention media and container materials after they are delivered to a construction site.
Another concern is the ability of bioretention systems, including conventional tree box filters, to process large quantities of fluid during peak flow periods without having backups that result in localized flooding of the surrounding areas. Most bioretention systems will have an upper limit for the amount of water that can be filtered at any time, as well as a maximum capacity for the amount of water that can be passed through the system in any event. Once the capacity of the system is exceeded, stormwater can pool at the surface of the planting soil.
It may be advantageous to incorporate one or more high flow bypass mechanisms into a tree box filter. To address stormwater flow during periods of peak flow and increase the upper limit for fluid flow, some bioretention systems employ an external high-flow bypass mechanism. The feature allows excess fluids to proceed through the drainage system without being filtered during periods of high fluid flow. This conventional high flow bypass is a separate structure, often a separate catch basin or similar device connected to the tree box filter by an external pipe or other mechanism and located downstream from the system. However, because the high flow bypass is an external structure—externally added to the tree box or other bioretention system components—its incorporation with tree box filters requires additional space (to accommodate the external bypass structure), as well as additional design, manufacturing, installation, and maintenance costs.
Another concern is the ability of the bioretention system to remove gross pollutants from incoming stormwater prior to releasing it. Ideally, the bioretention system should pre-treat (e.g., using filtration systems) water flow from the developed land prior to releasing it. The entrance of gross pollutants, such as trash, debris, floatables, and coarse sediments, are known to “clog” the system and thus reduce the efficiency. It also increases the maintenance frequency of typical bioretention systems. Pre-treatment apparatus that can remove gross pollutants from the treated flows should be incorporated into the bioretention system in order to minimize land usage. The pretreatment apparatus also should be accessible for intermittent cleaning, repair, and/or other maintenance.
Further, bioretention systems typically are installed under large concrete or asphalt surfaces to treat stormwater that has run over impervious surfaces in commercial, residential, and industrial areas such as median strips, parking lots, and sidewalks. They must be structurally sound and capable of bearing highly variable weight loads. It is desirable for the systems to maximize water storage while occupying as small a “footprint” as possible to minimize land usage and site excavation costs.
Accordingly, what is desired is a bioretention system solving some or all of the foregoing problems, including a bioretention system that can allow for stormwater infiltration into the surrounding soil, while reducing material loading, transportation, and storage costs. It is another objective of the invention to provide for a bioretention system that can effectively process increased amounts of stormwater runoff during peak periods of high fluid flow and can efficiently utilize space within a developed land site. It is another objective of the invention to provide a bioretention system that has pre-filtration capabilities to remove gross pollutants from stormwater runoff before it is released. It is yet another objective of the invention to provide a flexible and economical design that simplifies the design and construction of stormwater drainage systems in a landscape area.